Conventionally, a gamma camera is used for identifying the position where a radioactive substance is present in environments where radiation arrives from a variety of directions, for example, in a nuclear power plant, a nuclear fuel or spent nuclear fuel processing facility, or in nuclear emergency situations. As such a gamma camera, a radiation measurement device is proposed (see Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-85250
This radiation measurement device includes a multiple collimator having a plurality of holes that allow components of predetermined directions of gamma rays radiated from a radioactive substance to pass through, a fluorescent screen that converts gamma rays having passed through the multiple collimator to visible light, and a screening container that covers the multiple collimator and the fluorescent screen to reduce radiation noise. The radiation measurement device combines a gamma ray image and an image photographed by another camera. In this manner, the radiation measurement device can grasp the condition, size, shape, and position of the internal radioactive substance as a radiation image for an object to be measured.
However, such a radiation measurement device is disadvantageously very heavy because it detects gamma rays radiated from the radioactive substance. To be more specific, in an environment where radiation arrives from a variety of directions, the screening container needs to screen out gamma rays arriving from other directions for grasping the direction from which the gamma rays arrive. For screening out gamma rays, the screening container needs to use thick lead. Also, for detecting gamma rays, the fluorescent screen is required to have sufficient thickness and high density to prevent gamma rays from passing through.
For example, in the case of detecting gamma rays having an energy of 662 keV radiated from 137Cs and grasping the arriving direction thereof, a fluorescent screen 126 provided in the previous stage of a photomultiplier 27, and a screening container 125 covering these are as shown in FIG. 12. That is, in the case of screening out gamma rays at an efficiency of 98%, the screening container 125 has a thickness of about 34 mm when lead having a specific gravity of 11.3 is used. When NaI (diameter 50 mm) having a specific gravity of 3.7 is used as the substance, the fluorescent screen 126 will have a thickness of about 10 mm for detecting gamma rays at an efficiency of 8%. So assuming that the radiation measurement device 102 is fabricated to have such a format that the field of view is ±22 degrees by a single collimator 121 having only one hole that allows a predetermined direction component to pass through, and light emission of the fluorescent screen by gamma rays is read by a photomultiplier of 65 mm long, the weight is about 25 kg only by the screening container 125 and the fluorescent screen 126, which is heavy to carry.
If lead that forms the screening container were to be thinned or if the fluorescent screen were to be thinned, the radiation measurement device would be light in weight. However, in such a radiation measurement device whose weight is reduced in this manner, disadvantageously, the accuracy is significantly impaired. That is, if the thickness of lead of the screening container is thinned, the radiation measurement device will detect gamma rays arriving from directions other than the predetermined direction, whereas if the fluorescent screen is thinned, the sensitivity will be impaired due to increased transmission amount of gamma rays. Therefore, in detecting a radioactive substance radiating gamma rays, thinning the screening container is limited, and also thinning the fluorescent screen is limited due to the relation with sensitivity.